The present invention relates to a fuel cell system for a vehicle having a fuel cell arrangement that includes a plurality of fuel cells and is designed to emit an output voltage, an output current and an output power during operation, with each fuel cell being designed to emit a cell voltage during operation. A vehicle drive forms a primary load, while a plurality of secondary loads are also present. A control apparatus controls the primary and secondary loads. The invention also provides a corresponding method.
Fuel cell systems are used as mobile energy sources for vehicles and are a futuristic alternative to conventional drive concepts using internal combustion engines. However, the implementation of this alternative leads to different requirements for matching to daily use. While conventional internal combustion engines can provide virtually the total maximum output power without delay after being started, the performance data of fuel cell systems is highly dependent on their operating conditions, such as the pressure, temperature, etc. A further exacerbating factor is that operation of a fuel cell system requires multiple peripheral components, which are needed to condition the substances used, and which likewise must be supplied with power during operation. This wide range of requirements and constraints for operation of fuel cell systems means that particular effort must be devoted during development to the control and energy management of the fuel cell systems.
For example, Japanese patent document PAJ 2005190967 (Publication Number) relates to a fuel cell system that is controlled as a function of operating parameters. This document proposes a method for starting a fuel cell system (and a corresponding fuel cell system), with the output current of the fuel cell system being controlled as a function of temperatures measured at various positions in the fuel cell system.
One object of the present invention is to provide a method and apparatus which implement an intelligent monitoring strategy for operation of the fuel cell system, even in particular operating conditions.
This and other objects and advantages are achieved by the fuel cell system according to the invention, which is suitable and/or designed for integration in a vehicle. The fuel cell system comprises a fuel cell arrangement with a plurality of fuel cells, which are preferably organized to form fuel cell stacks, with the number of fuel cells in a fuel cell stack or in the fuel cell arrangement preferably being more than 100. In one preferred embodiment, the fuel cells have a PEM membrane and are designed to create an electrochemical reaction between a fuel (in the form of hydrogen) with an oxidant (in the form of environmental air), in order to obtain electrical energy from the chemical energy.
During operation, the electrical energy is emitted from the fuel cell arrangement as an output power, at an output voltage and an output current. Each fuel cell emits a cell voltage in accordance with its polarization characteristic, with the total of cell voltages (with fuel cells preferably connected in series) resulting in the output voltage of the fuel cell arrangement or stack.
The fuel cell system has a primary load which is in the form of a vehicle drive, and a plurality of secondary loads, which can be subdivided into at least two groups: A first group relates to the peripheral components which are absolutely essential to allow operation of the fuel cell arrangement. (For example, these peripheral components may include an air compressor, fuel recirculation fan, cooling water pump and the like.) Peripheral components such as these are also referred to as parasitic components or balance-of-plant (BOP). A second group, on the other hand, comprises loads which can be switched on optionally, such as DC/DC converters for supplying a high-voltage rechargeable battery or capacitor, a DC/DC converter for supplying low-voltage components, variable loads (electrical heating device) and the like.
The fuel cell system according to the invention has a control apparatus for controlling the primary and secondary loads, particularly power distribution to the primary load and the secondary loads. The power can be distributed by switching the primary load and/or the second loads on and off in a stepped form and/or continuously variably.
According to the invention, the control apparatus includes programming and/or circuitry which provide a monitoring circuit in a special operating mode of the fuel cell system. For the purposes of the monitoring circuit, the secondary loads are switched on and/or off and controlled as a manipulated variable, such that the output voltage (as a reference variable) in the monitoring circuit is kept at a low voltage value that results from a cell voltage of the individual fuel cells of less than 0.45 V on average. In particular, the monitoring circuit may be in the form of a closed loop and/or an open loop. In modified embodiments, it may also be implemented for the purposes of a neural network, fuzzy logic, adaptive regulators, etc.
During special operating modes of the fuel cell system, it may be technically important to keep the fuel cells at a low cell voltage. This situation occurs, for example, during so-called cold and/or freezing starting of the fuel cell system, when the operating temperature of the fuel cells is less than the normal operating temperature of about 80° C. (in particular is less than 0° C.). In this special operating mode, it is desirable to actively increase the temperature of the fuel cells. It has been found that the self-heating of the fuel cells by heat losses is approximately inversely proportional to the cell voltage of the fuel cells. It is therefore advantageous for a rapid heating process for the cell voltages of the fuel cells to be kept as low as possible. Cell voltages of less than 0.45 V (particularly in a range between 0.2 and 0.45 V) have been found to be advantageous in this case. In order to obtain this low cell voltage, and the low resultant value of the output voltage, according to the invention, the secondary loads are switched on in a dynamically stepped or infinitely variable manner. In this manner, the output voltage is reduced to the said low voltage value by increasing the output current.
In one preferred embodiment of the invention, the control apparatus is designed to keep the output voltage at a constant value as the low voltage value. This embodiment has the advantage that the operation of the individual fuel cells is not adversely affected by widely fluctuating cell voltages.
The special operating mode is preferably cold starting or freezing starting and/or is arranged for an instantaneous output power from the fuel cell arrangement, in which case, although the output power is greater than the necessary self-supply power for the first group of secondary loads, it is, however, preferably less than the maximum secondary load power.
The maximum secondary load power is the power which can be achieved when all available secondary loads are connected to the fuel cell arrangement and are being operated at maximum load. For an example of a fuel cell system whose normal output voltage is 250 V to 450 V, the output current may extend up to 200 Amperes without moving the vehicle (that is, without current and power being emitted to the primary load).
Analogously, a maximum secondary load current is the current value which can be reached when all the available secondary loads are connected to the fuel cell arrangement and are being operated at maximum load at the low voltage value. The special operating mode occurs in this example when the output current is greater than the self-supply current for the peripheral components, but is less than the maximum secondary load current, in each case at the low voltage value.
In one preferred embodiment of the invention, the control apparatus is designed such that the output voltage is greater than the low voltage value in a range between zero power or zero current (that is, when the fuel cell system is being started) and a self-supply power or current. This development of the monitoring strategy provides for the output current or the power consumption to be increased in steps until the fuel cell arrangement is loaded such that the desired low voltage value occurs.
In a further optional embodiment of the invention, in an output power range in which the output power is greater than the maximum secondary load power and/or is greater than the self-supply power, or a corresponding output-current range, the control apparatus is designed to monitor the stoichiometry of the oxidant-fuel ratio as a further manipulated variable. Preferably, the stoichiometry is set by an air compressor, fan or the like.
The stoichiometry is actually the ratio of the amount of substance supplied and the amount of substance converted in the reaction in accordance with Faraday's Law, for a respective reaction partner. Both the oxidant and the fuel therefore each have their own stoichiometry values. However, in practice, the expression stoichiometry is very often used, as in the following text as well, for the ratio of these two stoichiometry values. The “stoichiometry” (lambda) in the following text therefore refers to the ratio between the oxidant and the fuel, with stoichiometry of lambda=1 defining equilibrium between these flows, a value of lambda <1 indicating a “rich ratio” with excess fuel, and a value of lambda >1 indicating a “lean ratio” with excess oxidant. The stoichiometry can be used on the one hand to control the output power of the fuel cells and on the other hand likewise to control the cell voltage, with values of lambda <1 reducing the cell voltage, and values of lambda >1 increasing the cell voltage.
The aim is therefore preferably for the manipulated variable “secondary loads switched on or off” and the manipulated variable “stoichiometry” to be controlled jointly, in order to keep the output voltage value at the low voltage value.
In one possible embodiment of the invention, the control apparatus is designed to emit a driving enable signal as soon as the available output power is greater than the self-supply power plus a variable power margin. This embodiment allows the control apparatus to assign sufficient power to the primary load, with the power output to the secondary loads being reduced at the same time in order to keep the output voltage at or below the low voltage value. The power assigned to the primary load is at least sufficient for careful or emergency operation of the vehicle at reduced power. In this case, the control apparatus is preferably designed to treat the power drawn by the driving operation as a disturbance variable in the monitoring circuit.
The invention also relates to a method for monitoring a fuel cell system, in which, in a special operating mode (for example, during cold or freezing starting), the output voltage of the fuel cell arrangement is kept at a low voltage value by switching secondary loads on or off. This low voltage value is less than the output voltage value which is formed by operation of the fuel cells in the fuel cell arrangement with a cell voltage of less than 0.45 V. This method therefore represents normal use of the fuel cell system just described.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.